TRANSMISSION JUMPS, D LIGHTS ON AND

I got a Montero Sport '99, after driving for 10 or 15 minutes, for example I stop on a stop sign and its like the car is in neutral and the transmission jumps when I acellerate to continue driving. If I drives over 80 mph for more than 30 minutes the D lights is on and the N light start flashing. What to do rebuild or replace the transmission?